1. Field of the Invention
The present invention relates to a boundary acoustic wave device including an electrode structure arranged at the interface between a piezoelectric substrate and a dielectric layer, and more specifically, to a boundary acoustic wave filter including a ladder circuit configuration.
2. Description of the Related Art
It is required that a band pass filter used in cellular phones have a large amount of attenuation in bands outside of a passband. In the past, a surface acoustic wave filter device has widely been used as such a band pass filter.
FIG. 10 is a circuit diagram showing a surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2003-101384. The surface acoustic wave filter device 1001 includes a series arm that connects an input terminal 1002 and an output terminal 1003. On the series arm, series arm resonators 1004 to 1006 are connected in series. A plurality of parallel arms are connected to the series arm. Each of the parallel arms includes a parallel arm resonator 1007.
Inductors L are respectively connected between the input terminal 1002 and the series arm resonator 1004, and between the series arm resonator 1006 and the output terminal 1003. Inductors L are also respectively connected to the ends of the parallel arm resonators 1007. These inductors L represent inductance components of wiring lines, connection portions, or bonding wires between the surface acoustic wave resonator chips and a package on which the chips are mounted.
An inductor Lp shown in FIG. 10 represents an inductance component of the portions from the connection pads on the package to the ground electrodes of an apparatus where the surface acoustic wave filter device 1001 is mounted, in other words, ground electrodes 1008 and 1009 shown in FIG. 10.
In the surface acoustic wave filter device 1001, a capacitor 1010 is connected between the end of the parallel arm closest to the input terminal 1002 and the parallel arm closest to the output terminal 1003.
The attenuation in a band lower than a passband can be increased while maintaining the width of the passband, by making the inductance component Lp extremely small and selecting an appropriate value for the inductance component L. However, in actual surface acoustic wave devices, it is very difficult to make the value of the inductance component Lp small due to restrictions on wiring of lines and other circuit limitations. Thus, in the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2003-101384, it is possible to increase the attenuation outside of the passband without decreasing the inductance component Lp by providing the capacitor 1010 described above.
On the other hand, a boundary acoustic wave filter device, which has a reduced size as compared to a surface acoustic wave device, has recently drawn attention as an alternative to a surface acoustic wave filter device.
In the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2003-101384, the attenuation outside of a passband is increased by providing the capacitor 1010. However, such a configuration is effective when a structure having a high inductance, such as a bonding wire, is connected to the ground terminal of the parallel arm resonator in a ladder filter.
On the other hand, in a boundary acoustic wave device, an electrode structure for exciting boundary acoustic waves is arranged at the interface between a piezoelectric layer and a dielectric layer and embedded in a boundary acoustic wave device chip. Thus, a package is not required, and the wiring lines do not have a large inductance component. Consequently, the method of increasing the attenuation outside of a passband by using the inductance component L and the capacitor 1010 described in Japanese Unexamined Patent Application Publication No. 2003-101384 is not effective for a boundary acoustic wave device.
Although there may be a method of providing an inductance component and a capacitance component by connecting an external device, this method will significantly diminish the advantage of a reduced size of a boundary acoustic wave device.